A device of this general type is described in our U.S. Pat. No. 4,549,614. The device described in this document has been tested and has yielded very good results. Nevertheless, it has become apparent that these results could be improved still further by modifying the shape of the counter-reaction member and its position in relation to the cutting disc. The counter-reaction member described in U.S. Pat. No. 4,549,614 is either a roller or a friction member which bears primarily on the cylindrical wall of the drilled hole in order to compensate the radial component of the reaction of the ground on the cutting disc. This centers the head and strengthens the cylindrical wall of the hole. However, depending upon the nature of the ground, the cutting head sometimes tends to enter too far into the ground, under the effect of the load to which it is subjected, and this can cause an overload or even a seizure of the means producing the rotation of the drilling head. Also, the head can deviate from the required drilling owing to slippage of the device on the ground at the bottom of the hole, this bottom not having been compacted after the disc has passed through.
The purpose of the present invention is to overcome these disadvantages by providing a drilling device having a counter-reaction member which eliminates the above-mentioned disadvantages by virtue of its shape and its arrangement.
The drilling device according to the invention is principally characterized in that the counter-reaction member is arranged so as to be in contact with only the concave bottom of the hole and so that, with respect to the axis of rotation of the body, the maximum radial projection of any part of the counter-reaction member and its means of attachment to the body is less than the maximum radial projection of the teeth on the cutting disc. This maximum radial projection of the counter-reaction member is therefore less than the radius of the hole generated by the device. As a result, the highest point of the surface of contact between the counter-reaction member and the hole is situated below the line at which the curved bottom of the hole meets the cylindrical wall of the hole. Also, the lowest point of the surface of contact between the counter-reaction member and the hole is separated from the axis of rotation of the body by a distance which is preferably, but not essentially, greater than the distance from the lowest point of the cutting disc to the same axis. Furthermore, the surface of contact of the counter-reaction member is convex and closely matches the concave surface of the bottom of the hole.
The advantages of the device according to the invention are the following: since the counter-reaction member is arranged on the body so as to be in contact with only the concave bottom of the hole, the withdrawal of the drilling device from the hole is made easier because, between the counter-reaction member and the cylindrical wall of the hole, there is a space. This space may also be useful in providing passage for the removal of the debris from the hole by means of the drilling fluid.
A further advantage of the device is that the contact between the counter-reaction member and the bottom of the hole serves to remove any irregularities in the concave shape of the bottom left by the previous passage of the cutting disc, by smoothing off any projecting peaks and compacting the surface of the hole in readiness for the next passage of the cutting disc. This action of reshaping and compacting the concave surface of the bottom of the hole reduces the risk of deviation from the required drilling axis. If the drilling device is not fitted with a counter-reaction member to perform this function, it may be subject to deviation from the drilling axis caused by diversion of the cutting disc from its true path by peaks and hollows in the irregular bottom of the hole or by slippage of the cutting disc into pockets of lower resistance in an uncompacted hole bottom.
Yet a further advantage of the device, according to a preferred embodiment, is that the surface of contact of the counter-reaction member is so positioned in relation to the cutting disc that it prevents the teeth on the disc from entering the ground at the cutting face by more than a predetermined depth. An efficient, steady and quick drilling operation is best achieved when the cutting teeth on the disc are constrained to partial entry into the ground, rather than to the full depth of the teeth, because, on the one hand, the debris cut away is smaller and thereby easier to remove by means of the drilling fluid, and on the other hand, it is known that the cutting action of the teeth is more efficient and requires the application of less force, thereby preventing the overloading of the means for driving the device in rotation.
The surface of the counter-reaction member in contact with the concave bottom of the hole may be so shaped that it maintains continuous contact with the hole over its entire area. In this form, the counter-reaction member may be made from a material, such as hardened steel, which is suitably resistant to withstand the abrasion resulting from contact wth the bottom of the hole during operation.
In another arrangement, the surface of the counter-reaction member may have interspersed on it a pattern of hard studs, as might be made from tungsten carbide, which project from the surface of the counter-reaction member by a typical distance of 2 mm or more. This provides flow-through channels for the flushing fluid between the surface of the counter-reaction member and the bottom of the hole for purposes of cooling the counter-reaction member and for the removal of fine debris from the cutting face. These studs might be arranged in an alternate overlapping pattern so as to provide a complete sweeping effect over the bottom of the hole between the highest and lowest points of contact of the counter-reaction member, thereby performing the aforementioned smoothing operation of the hole bottom most effectively. Alternatively, in some types of ground, it may be preferable to arrange the studs in spaced pairs with the studs of each pair being radially aligned with one another from the axis of rotation of the drilling device. This pattern provides uninterrupted flow-through channels in the space between each pair of studs for efficient cooling and debris removal.
In a preferred arrangement, the rotational axis of the cutting disc relative to the body is displaced from the axis of rotation of the device, so that the cutting disc is positioned to provide some "lead" in the direction of rotation by means of a concentric arrangement. This displacement ensures that cutting is performed by the leading teeth on the disc and that clearance exists between the trailing teeth and the hole to prevent the stalling of the disc in rotation. Such stalling might be caused by engagement of the trailing teeth with the hole if "lead" were not provided. This displacement also facilitates removal of debris in the clearance thus created behind the disc. With this arrangement, cutting is performed by the teeth in the lower leading, or cutting, quadrant of the disc and, accordingly, the preferred position of the counter-reaction member is diametrically opposite to the cutting quadrant through the axis of rotation of the device. In this position, the counter-reaction member is best equipped to resist and counteract the radial forces imposed by the ground on the device at the cutting face and thus to prevent deviation or wander from the required drilling axis.
According to yet another embodiment, the directional stability of the drilling axis is further assured by the provision of two counter-reaction members radially positioned, typically, at 120 degrees with respect to each other and to the cutting quadrant on the disc. In its simplest and most economic arrangement, the counter-reaction member may be formed integrally with the body of the device, as could be produced by the inclusion of the shapes of both the body and the counter-reaction member in a single steel casting.
However, the surface of the counter-reaction member which contacts the bottom of the hole can be subject to severe wear during use of the device, and this surface may degrade to the point at which premature unserviceability of another usable body is caused. In a preferred embodiment, therefore, the counter-reaction member is provided as a separate component from the body and means are provided for location and assembly thereto. In the event that wear of the counter-reaction member progresses to the point of impairment of function, it can be removed from the body and replaced with a new counter-reaction member, to enable continued use of the drilling device.
Yet a further embodiment provides for the attachment of the counter-reaction member to the body in such a manner that the position of the counter-reaction member may be adjusted in relation to the cutting disc, in order to achieve the optimum position for efficient cutting. This may be done by mounting the counter-reaction member in slide locations formed in the body so as to be aligned with the axis of rotation of the device, and securing to the body by means of an adjusting screw. The height of the counter-reaction member relative to the cutting disc may then be varied to suit the nature of the ground being drilled.